The Spring one linked above is not really comparable to Django admin in my opinion. In particular it doesn’t integrate with your data model at all - it essentially just shows actuator endpoints.
Super-simple to set up and it's been perfectly adequate for my app's needs so far.
To answer OP's original question: Django Admin isn't a killer feature because the same kind of thing is available for most other Django-like frameworks, with the only difference being that it's usually a third party library rather than something built into the framework itself.
Being built into the framework is a big deal, though.
The problem with third-party libraries for this kind of thing is that there's no guarantee that they will be maintained in lockstep with the underlying framework.
What if they get abandoned? Or a new version of the framework comes out with new features but the third-party library takes months to add support for them?
I think this may be an aspect of Django that isn't appreciated nearly as much as it should be: batteries included means that the feature you depend on are guaranteed to be maintained and documented at the same pace as the rest of the framework.